text: LAGP is a solid ionic conductor and features the two fundamental properties to be used as a solid-state electrolyte in lithium-ion batteries, namely a sufficiently high ionic conductivity and a negligible electronic conductivity. Indeed, during battery operations, LAGP should guarantee the easy and fast motion of lithium ions between cathode and anode, while preventing the transfer of electrons.
  As stated in the description of the crystal structure, three kinds of sites are available for hosting lithium ions in the LAGP NASICON structure, i.e. the Li(1) sites, the Li(2) sites and the Li(3) sites. Ionic conduction occurs because of hopping of lithium ions from Li(1) to Li(2) sites or across two Li(3) sites. The bottleneck to ionic motion is represented by a triangular window delimited by three oxygen atoms between Li(1) and Li(2) sites.
  The ionic conductivity  in LAGP follows the usual dependency on temperature expressed by an Arrhenius-type equation, which is typical of most of solid-state ionic conductors:
  where
  :  is the pre-exponential factor,
  :  is the absolute temperature,
  :  is the activation energy for ionic transport, 
  :  is the Boltzmann constant.      
  Typical values for the activation energies of bulk LAGP materials are in the range of 0.35 - 0.41 eV. Similarly, the room-temperature ionic conductivity is closely related to the synthesis conditions and to the actual material microstructure, therefore the conductivity values reported in scientific literature span from 10 S/cm up to 10 mS/cm, the highest value close to room temperature reported up to now. Compared to LGP, the room-temperature ionic conductivity of LAGP is increased by 3-4 orders of magnitude upon partial substitution of Ge by Al. Aluminium ions have a lower charge compared to Ge ions and additional lithium is incorporated in the NASICON structure to maintain charge balance, resulting in an enlarged number of charge carriers. The beneficial effect of aluminium is maximized for x around 0.4 - 0.5; for larger Al content, the single-phase NASICON structure is not stable and secondary phases appear, mainly AlPO, , and GeO. Secondary phases are typically nonconductive; however, small and controlled amounts of AlPO exert a densification effect which affects in a positive way the overall ionic conductivity of the material.
  The prefactor  in the Arrhenius equation can in turn be written as a function of fundamental constants and conduction parameters:
  where
  :  is ion valence,
  :  is the elementary charge,
  :  is the absolute temperature, 
  :  is the Boltzmann constant,
  : is the concentration of charge carriers,      
  : is the average velocity of the ions,
  : is the mean free path.       
  The prefactor is directly proportional to the concentration of mobile lithium-ion carriers, which increases with the aluminium content in the material. As a result, since the dependency of the activation energy on aluminium content is negligible, the ionic conductivity is expected to increase with increasing Ge substitution by Al, until secondary phases are formed. The introduction of aluminium also reduces the grain boundary resistivity of the material, positively impacting on the total (bulk crystal + grain boundary) ionic conductivity of the LAGP material.
  As expected for solid ionic conductors, the ionic conductivity of LAGP increases with increasing temperature.
  Regarding the electronic conductivity of LAGP, it should be as low as possible to prevent electrical short circuit between anode and cathode. As for ionic conductivity, the exact stoichiometry and microstructure, strongly connected to the synthesis method, have an influence on the electronic conductivity, even if the reported values are very low and close to (or lower than) 10 S/cm.